1. Field of the Invention
The invention relates to a conference system for establishing a simultaneous interchange of information among a plurality of subscribers, comprising a conference circuit for information signals of the conference, a connecting circuit for connecting each participant to the exchange of information on the conference circuit, each connecting circuit being formed by an output circuit for transmitting the information from a participant to the conference circuit and an input circuit for receiving the information from the conference circuit for said participant, the conference circuit being arranged for receiving input information signals from the output circuit of each of the connecting circuits and for applying output information signals to the input circuit of each of the connecting circuits, said output information signals comprising the algebraic sum of the input information signals of each of the further output circuits with a predetermined phase and a predetermined amplitude ratio.
2. Description of the Prior Art
Such a conference system is disclosed in U.S. Pat. No. 4,049,920. The output information signal which is transmitted from the conference circuit to the connecting circuits of each individual participant comprises, as stated in the foregoing, the algebraic sum of the input information signals of the further participants. The phase of these input information signals (the sign of the contribution to the algebraic sum) is empirically chosen so that instabilities due to oscillation are prevented to the maximum possible extent: the output information signals are transmitted to the participants where they are partly retransmitted due to hybric circuit impefections. A disadvantage of this system is that the insertion losses of the conference circuit are comparatively high.
The invention has for its object ot provide a conference system of the type described in the foregoing which, while maintaining absolute stability has lower losses. The invention provides a conference system as described in the opening paragraph characterized in that the phase is chosen so that the matrix S which represents the said algebraic relationship between the output information signals and the input information signals is defined by the relation E-S*S 0, wherein E is the unity matrix and S* is the transposed complex conjugate matrix of S.
It should be noted that reference (1) mentioned at the end of the description states (.sctn. 3.2) that ideal two-wire conference circuits can be designed which are assembled from ideal transformers. Conference circuits of that type are possible for 4N+2 participants, where N is an integer, i.e. for (2), 6, 10, . . . participants. FIG. 11 and FIG. 12 of the said publication show implementations of conference circuits for 6 and 10 participants, respectively. These implementations have been found possible as the S-matrix of the circuit is symmetrical, so that it is possible to realize an ideal conference circuit in the manner disclosed, using reciprocal network elements.
Reference (1) further states (.sctn. 3.4) that in theory a four-wire ideal conference circuit is also possible for 2N+2 participants. In that case, however, the S-matrix is not symmetrical, which means that inter alia non-reciprocal network elements are required for the realisation of such a conference circuit. However, an implementation of an ideal four-wire conference circuit is missing.